The input offset voltage of an operational amplifier is the voltage which must be applied between its input terminals in order to bring the output voltage to a predetermined reference level when there is no signal input. This offset voltage is a generally known error characteristic of operational amplifiers which is due to an uncontrollable mismatch of components.
For certain applications of operational amplifiers, it is particularly important to eliminate errors which result from uncompensated input offset voltages. Such is the case, for example, for operational amplifiers which are used for charge redistribution codecs in telephone communications, where it is generally desirable to maintain the effects of each internal source of error to less than one-tenth LSB (least significant bit). A particular charge-redistribution codec is described, for example, in Suarez et al., "An ALL-MOS Charge-Redistribution A/D Conversion Technique," 1974, IEEE Internat. Solid-State Circuits Conference, p. 194, U.S.A.
It is possible by means of various feedback loop arrangements to adjust the offset voltage on a continuing basis, such as for analog systems. However, such arrangements involve relatively complex circuitry including comparators.
Another and simpler approach is to compensate the offset voltage by means of a voltage source, such as a voltage compensating capacitor, which by means of electronic switching devices, which may be MOSFETS (metal-on-silicon field-effect transistors), is periodically reset to the offset voltage and then connected in series with the signal input to the amplifier. This approach is used in so-called "chopper-stabilized" amplifiers, such as is described, for instance, in U.S. Pat. No. 3,617,913 issued Nov. 2, 1971 to Rolf Schmidhauser et al. This type of stabilization is particularly suitable for digital circuits, since even for a high speed digital signal there is generally sufficient time between input signal pulses for resetting the compensating voltage of the capacitor. It is also an approach particularly suited for integrated circuits, since the required capacitors and MOSFETS can be easily provided in such structures and take up relatively little space. However, the switching devices themselves have an inherent stray capacitance which can introduce a feed-through charge onto the compensating capacitor in the course of their switching and thereby result in a compensating voltage error.
In typical offset voltage compensation arrangements, a compensating capacitor is connected in series with the signal input of the amplifier. Resetting of the capacitor's compensating voltage is accomplished by means of three MOSFET switches. An isolating switch disconnects the signal source from the capacitor, an input reset switch grounds the signal input terminal, and a feedback switch connects the disconnected side of the capacitor to the other feedback input terminal of the amplifier. Before the input signal source can be reconnected to the signal input terminal through the compensating capacitor, it is necessary to open the feedback switch which connects the capacitor to the other input terminal and to also open the input reset switch which grounds the signal input terminal. These switches generally cannot be operated precisely simultaneously and do not have identical switching characteristics, such as stray inherent capacitances.
If the feedback switch opens first, then the two switches will not see the same terminations during opening and will therefore inject different feed-through charges onto the capacitor to result in a substantial net feed-through error. On the other hand, if the input reset switch connecting the signal input terminal to ground opens first, then its feed-through error will be seen by the amplifier as a step input. The response of the amplifier to the step input will be transmitted back through the feedback loop to the other input terminal and to the capacitor, unless the feedback switch can be operated much faster than the response time of the amplifier. For very fast amplifiers, the switch will not be fast enough.
It is known that the charge feed-through error can be reduced by the provision of matching charge compensation devices for the MOSFET switches. The charge compensation devices, however, can themselves bring additional problems. In one approach, the charge compensation devices increase the silicon surface area used and further increases the open switch leakage. A large capacitor could accommodate the increased leakage, but it is desirable to make this capacitor as small as possible in order to minimize both its settling time and the circuit substrate area required for it. Other charge compensation approaches for the electronic switching devices have not led to a sufficient reduction in the offset compensating voltage error.